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Operating a servo from a transmitter switch

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Ross McWhirter05/08/2018 22:47:12
28 forum posts

I have a Multiplex Funcub XL that has bomb bay doors that can be opened with a servo operated by a transmitter switch. How do I set the servo movement so that flicking the switch opens the doors? I have a Futaba radio.

Edited By Ross McWhirter on 05/08/2018 22:48:30

Manish Chandrayan06/08/2018 01:32:57
537 forum posts
65 photos
Channel 5 on the Futaba should be the retract channel and operated from a switch. Have you tried hooking up the bomb bay servo to this channel?
Percy Verance06/08/2018 06:37:10
7567 forum posts
145 photos

Hi Ross

I also have a Funcub XL. It's a great model, but I'd urge a note of caution regarding dropping anything from the model. It might be best to check your insurance documents as you may not be covered for such activities.

Many show organisers ceased children's sweet dropping displays years ago, should anyone get hit in the eye by a falling sweet........

Cuban806/08/2018 07:47:50
2357 forum posts
8 photos
Posted by Percy Verance on 06/08/2018 06:37:10:

Hi Ross

Many show organisers ceased children's sweet dropping displays years ago, should anyone get hit in the eye by a falling sweet........

Simply have the sweets attached to long streamers and drop them before the kids are allowed into the drop area. Model clears and kids pick the sweets up from the ground. What on earth is dangerous in that?..........risk of picking up an infection from sweets on the ground..........might fall over running for the sweets..........emotional distress at not getting a sweet (organisers to provide a safe space and disappointment counselling).

What a joke.

Former Member06/08/2018 07:51:51

[This posting has been removed]

simon barr06/08/2018 08:00:41
1020 forum posts
11 photos

Hello Ross.

I think the answer you are looking for is connect the bomb door servo to a switch that is convenient for you, although on an entry level transmitter this will be limited, so you will probably end up using channel 5, the retract switch.

Then you may need to reverse the switch function so that the doors operate with the switch position you want (flick down for open and up for up).

If there is too much travel on the servo which forces too much travel on the bomb doors, then you need to start adjusting the throws with the end point setting.

Hope this makes sense and helps,

Howard Winwood06/08/2018 08:53:35
10 forum posts

with regard to servo movement, it will be at one end of it's travel with the switch in one position and the oposite end of it's travel when the switch is operated. once you know what the total travel is, you can adjust the linkage to suite the travel you need. it may also be able to set the travel limits in the transmitter. most modern computer TX,s can do this via on screen menu.

Frank Skilbeck06/08/2018 09:18:24
4310 forum posts
101 photos

I'd start by winding down the servo movement to almost nothing and then connecting the doors with them open halfway, you can then adjust the end point travel with switch in the closed and open positions to get the correct shut and open positions, this way you won't stall the servo if the 100% travel setting is too much.

The Wright Stuff06/08/2018 10:45:36
1383 forum posts
226 photos

Hi Ross,

I think it's important to know which Futaba radio you have. It rather depends upon how many channels it has, and different models allow you to configure the switches differently.

Sorry if this is obvious, but not all the switches can be used to move servos (i.e. they are not all independent channels), some can only be used for buddy boxes, switching rates, etc...

best wishes,


Ross McWhirter06/08/2018 13:49:18
28 forum posts

Thanks everyone for the replies. My radio is a Futaba T8FG. I'll connect to channel 5 and follow your advice.

My idea for fun is to drop a toy soldier parachutist over our flying field.

ROSS MANSELL 106/08/2018 14:00:58
7 forum posts

Thought it was illegal to drop anthring from a model??? ANO??

Geoff Sleath06/08/2018 14:41:11
3201 forum posts
247 photos

Well, at least one model dropped parachutes at Cosford this year. Most of them landed in front of the spectators but one floated away downwind occasionally gaining height It wasn't a problem.


Martin Harris06/08/2018 14:56:04
8301 forum posts
210 photos
Posted by ROSS MANSELL 1 on 06/08/2018 14:00:58:

Thought it was illegal to drop anthring from a model??? ANO??

Not so - it is only prohibited if it causes endangerment:

"A person must not cause or permit any article or animal (whether or not attached
to a parachute) to be dropped from a small unmanned aircraft so as to endanger
persons or property."

Keith Lomax09/08/2018 00:11:46
198 forum posts

It is possible to rig the linkage such that the servo pulls a rod through a loop of some sort; then a fixed fastening on the rod pulls the loop for only part of the movement, the loop is then part of the linkage to the doors. This is how some RC cars do the brakes.

Denis Watkins09/08/2018 08:35:57
3495 forum posts
165 photos

Draw it Keith

I experiment all the time with these mechanisms

Especially with sprung undercarriage linkages, and airbrake doors

But each hole you make, each pivot, and rod has a tiny bit of slack that adds up to inaccuracy and friction

Your job is to minimise failure

Keith Lomax09/08/2018 09:37:44
198 forum posts

Thinking more about it, they do it so that for a car with an IC engine (and therefore no reverse), what would have been reverse applies the brakes.

I googled "rc car brakes throttle servo" and this was the first picture that I found - but the link didn't paste very well.

eflightray09/08/2018 10:36:59
560 forum posts
127 photos

Try a Google search on -- rc plane parachute release -- loads of information including videos.


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